Presently commercially available ethylene-vinyltrimethoxysilane copolymer resins having a vinyltrimethoxysilane (VTMS) content in the range of about 1.0-2.0 weight % and a density below 925 kg/m3 are not suitable for pipe applications within the segment of plumbing and heating. Such water crosslinkable polyolefin pipes (PEX-b) do not have the properties required to pass the quality control point of hydrostatic pressure resistance at 95° C. in accordance with i. e. the German medium density norm for PEX pipes, DIN 16894. The lifetime of a pipe according to this norm should be at least 1000 hour at 95° C., the circumferential stress in the pipe wall being 2.8 MPa. At a temperature as high as 95° C. the hydrostatic pressure resistance of the commercially available pipe above is very week and such a pipe has a lifetime of less than 1 hour, i. e. the time to failure is less than 1 hour.
Attempts have been made in order to improve the hydrostatic pressure resistance of pipes made of ethylene-vinyltrimethoxysilane copolymer resins by increasing the density of the copolymer resin by mixing the resin with a high density polyethylene polymer (HDPE, i. e. PE made by low pressure polymerization). However, incorporating about 30 weight % high density polyethylene polymer into the resin and thereby increasing the density thereof to 934 kg/m3 does not result in an increased lifetime. Accordingly such pipes will not pass the quality control of DIN 16894.
Ethylene-vinylsilane copolymers, such as e. g. Visico™, may also be used for moisture crosslinking within the field of cable insulations. However, the prior art silane crosslinked polyethylene insulation materials have exhibited problems with so called “frozen layers”, i. e. the molecules of the co-polymer will not have the time to relax when they hit the cold conductor and this will in turn lead to a thin layer of highly oriented molecules being formed close to the conductor. Such orientation of the molecules leads to deterioration of the mechanical properties. Previously, this problem has been solved by preheating the conductor or by decreasing the shear stresses by using a tube on die in stead of a pressure die. However, the investment in a preheater induces higher costs. Furthermore, a reduction of the die pressure by using a tube on die leads to a deterioration of the wetting properties, and this in turn involves reduced adhesion properties between the conductor and the insulation material.